1. Field of the Invention
This invention relates to a method of measuring physical properties of a buried channel formed under the surface of a semiconductor substrate, including the channel potential and the surface potential of the buried channel, a generation current in the semiconductor substrate and another generation current on the surface of the substrate.
2. Description of Related Art
FIG. 5 shows the structure of a buried-channel MOS transistor 10 wherein an n.sup.- layer buried channel 2 is formed at the surface of a p-type semiconductor substrate 1. N.sup.+ contact portions 3 and 4 are formed at opposite ends of the buried channel 2. The upper surface of the buried channel 2 is covered with an insulation film 5. A gate electrode 6 is formed on the insulation film 5. This MOS transistor operates as a gate controlled diode with a source terminal 7 and a drain terminal, 8 respectively connected to the contact portions 3 and 4 that are connected as a common electrode to a reverse bias voltage V.sub.R FIGS. 6A and 6B show conventional measuring systems for measuring physical properties of the buried channel 2 with the semiconductor device 10 having this gate controlled diode structure.
The system shown in FIG. 6A measures a generation current inside the semiconductor substrate 1 together with a generation current at the surface thereof, and includes a small current measuring apparatus 11 and a CPU 12. The system shown in FIG. 6B measures the potential of the buried channel 2, the impurity density distribution therein and includes a low-frequency CV measuring apparatus 13, a power source 14 and a CPU 15. These systems are used as described below.
A reverse bias voltage V.sub.R is applied to the contact portions 3 and 4 of the semiconductor device 10 shown in FIG. 5. Changes in a reverse bias current I.sub.R are measured by the small current measuring apparatus 11 shown in FIG. 6A during the sweeping of a variable voltage gate voltage V.sub.G applied to the gate electrode 6. FIG. 7A shows results of this measurement. As described by A. S. Grove and D. J. Fitzgerald in Solid State Electronics, Volume 9, 1966, pp 783-806, a generation current Igen,.sub.MJ inside the semiconductor substrate 1 and a generation current Igen,s at the surface of the semiconductor substrate 1 can be obtained from the results shown in FIG. 7A.
The reverse bias voltage V.sub.R is applied from the power source 14 of the system shown in FIG. 6B to the contact portions 3 and 4 of the semiconductor device 10 shown in FIG. 5. A low-frequency gate voltage V.sub.G is applied from the low-frequency CV measuring apparatus 13 to the gate electrode 6, thereby measuring the low-frequency CV characteristics and obtaining results such as those shown in FIG. 7B. In FIG. 7B, C.sub.OX represents the capacitance of the insulation film 5. According to A. M. Mohsen and F. J. Morris: Solid State Electronics, Volume 18, 1975, pp 407-416, it is possible to calculate from the results shown in FIG. 7B a channel potential .phi..sub.CH, a surface potential .phi..sub.s, and a depth X.sub.CH from the surface to the position of the channel potential .phi..sub.CH with respect to the diagram of a potential distribution over the buried channel shown in FIG. 8. It is also possible to obtain an impurity density distribution between the surface and the channel position depth X.sub.CH by using low-frequency CV characteristics between a voltage V.sub.R +V.sub.FB and a punch-through voltage V.sub.PT with respect to the depletion layer. V.sub.FB represents a flat-band voltage.
A method of calculating the channel potential .phi..sub.CH and the surface potential .phi..sub.s on the basis of the method of A. M. Mohsen and F. J. Morris will be described below with reference to the flow chart of FIG. 9. First, in step 91, results of measurement, such as those shown in FIG. 7B, obtained by the measuring system of FIG. 6B are displayed on a CRT (not shown) connected to the CPU 15. In step 92, the punch-through voltage V.sub.PT is input. In step 93, the channel potential .phi..sub.CH is calculated on the basis of an equation: EQU .phi..sub.CH =V.sub.R -Vbi [1]
where Vbi represents a built-in potential of the buried channel 2 and the contact portions 3 and 4.
The surface potential .phi..sub.s is represented by an area S.sub.A of a hatched region A of FIG. 7B, and is calculated, in step 94, on the basis of an equation: ##EQU1## where V.sub.INV represents the voltage on the inversion side.
The characteristics of the buried channel 2 are thus obtained. However, as described above, the conventional method requires two systems of different construction: one for measuring the generation current Igen,.sub.MJ in the semiconductor substrate 1 and the generation current Igen,s of the surface of the semiconductor substrate 1; and one for measuring the channel potential .phi..sub.CH, the surface potential .phi..sub.s, the depth X.sub.CH from the surface to the channel position and the impurity density distribution. These characteristic factors cannot be measured at one time and they are necessarily measured separately by using such systems. The construction of the overall system is therefore complicated, and the measuring operation is time-consuming and troublesome.
If the system of FIG. 6A and the system of FIG. 6B are combined to measure all the characteristic factors at one time, noise is superposed on the very small reverse bias current I.sub.R measured by the small current measuring apparatus 11 owing to oscillations of the low-frequency voltage used in the low-frequency CV measuring apparatus 13. As a result, the accuracy with which the generation current is measured reduced.
Since, at the time of full depletion, the depletion layers at the contact portions 3 and 4 contact the depletion layer at the buried channel 2 the influence of the capacitance at the periphery of the channel on the characteristic curve of the measured CV characteristic is usually considerable, as represented by hatched portion D in FIG. 10. An error proportional the area of the hatched portion D of FIG. 10, is caused in the calculation of the surface potential .phi..sub.s based on the equation [2] and using the CV characteristic curve measured by the low-frequency CV measuring apparatus 13. As a result the potential is not accurately measured.